Flickr was a game but users drove it into a photo sharing site.

The typical example of a socio-technical system heavily influenced by its user (prosumer!) is Flickr. Maybe not too many people know that in the beginning Flickr was a Web-based game, called Game Neverending (GNE). Best summary of early Flickr history I found is in this interview.

The original interface of GNE (see below) was heavily based on Instant Messagging. You could drag game objects into an IM conversation and it would send to all the other members of the chat an image of the object.
THAT was the key feature! The creators of GNE thought “what if instead of game objects, you could drag and drop other digital objects into these conversations, like Word documents, or PDFs? Or maybe photos?”
So the first version of Flickr was just a stripped-down Game Neverending interface, with photos instead of game objects.

I think this is the perfect example of user-driven design. “I created a web game site –> users use it for sharing objects –> then I create a site for sharing photos.”